1. Field of the Invention
The present invention relates to a honeycomb monolith heater comprising a honeycomb structure having a plurality of electrode plates fixed to an outer wall of the honeycomb structure.
Honeycomb heaters of the above-described type can be employed as heaters for domestic use, such as a hot-air heater, or as industrial heaters, such as preheaters used for control of automobile exhaust emissions.
2. Description of the Prior Art
These days, besides porous ceramic honeycomb structures conventionally employed, metal honeycomb structures are drawing attention as catalysts or catalyst carriers for removing nitrogen oxide, carbon monoxide and hydrocarbons present in the exhaust gas from internal combustion engines, such as from automobiles.
As restriction of exhaust emission has been intensified, there has been a demand for development of heaters for use in reducing automobile exhaust emission when the temperature of the catalyst is not sufficiently high, i.e., at the beginning of running of a vehicle.
Honeycomb structures have been proposed in, for example, U.K. Patent 1492929 and Japanese Utility Model Laid-Open No. 67609/1988.
U.K. Patent 1492929 discloses a foil-type metal honeycomb structure in a catalyst for use in automobile exhaust emission control. This honeycomb structure comprises a metal substrate produced by winding, together with a flat plate, a mechanically deformed, corrugated flat plate. This metal substrate has an aluminum oxide film formed on the surface thereof by an oxidation process. The catalyst for use in automobile exhaust emission control is manufactured by placing a large surface area oxide, such as alumina, on the aluminum oxide film of the metal substrate and by impregnating a noble metal on the large surface area oxide.
Japanese Utility Model Laid-Open No. 67609/1988 discloses an electrically conductable metal monolith catalyst for use as a preheater comprising a metal support and alumina coated thereon.
In the foil-type metal honeycomb structure disclosed in U.K. Patent 1492929, however, telescope phenomenon readily occurs during the run cycle in which metal-to-metal joins break and the metal substrate is deformed in such a manner that it protrudes in the direction of the flow of gas. This may disturb safe running of the vehicle. Furthermore, in the manufacture of the foil-type metal honeycomb, yield of the rolling process is low, inviting high production cost. In the preheater proposed in Japanese Utility Model Laid-Open No. 67609/1988, a catalyst readily peels off due to a difference in thermal expansion between alumina and the metal support. Furthermore, metal-to-metal joins of the metal substrate break during operation, generating an electrically insulating portion and, hence, non-uniform flow of current and non-uniform heating.
Furthermore, in the preheater disclosed in Japanese Utility Model Laid-Open No. 67609/1988, since the electrodes are provided on the inner peripheral portion of the preheater, the central portion thereof does not act as a catalyst and pressure loss may be generated. Furthermore, the electrodes readily breaks due to the flow of gas.